Conventional central air conditioning and heating equipment comprises a blower which forces air through a heat exchanger and thereafter distributes the air through a system of air distribution ducts or plenums to various parts of a building. Similarly such systems include a return path also comprising ducts for returning air from within the building to the heat exchanger. Optionally the system of ducts may include ducts for introducing fresh air from outside the building into the system.
As such heating and air conditioning system have become larger to accommodate larger volumes in larger buildings, cross sections of the air circulating ducts have become progressively larger, and it has often become necessary to direct some of the larger ducts outside the building. With most industrial air conditioning and heating systems currently being located on the roof of buildings a substantial effort has been expended to reduce the heat transfer characteristics particularly of exposed duct work, as illustrated by numerous patents on duct insulation schemes, including U.S. Pat. Nos. 4,735,235 issued to Anderson et al. and U.S. Pat. No. 6,360,783 issued to Faviero, IV et al.
While insulating the ducts as taught by these patents provides substantial improvement in reducing the heat losses in such duct systems, particularly when such ducts are located within a building structure and therefore protected from the elements, ducts located on building roofs or other exposed areas present a host of additional problems due to their exposed nature, such as water or snow accumulation on their top surface. This problem is particularly vexing since the large ducts used on roof tops often have a square or rectangular cross section, such cross sections offering other unrelated advantages over circular cross section ducts. Such accumulation adversely effects the heat exchange characteristics, water being a better heat conductor than air, promotes oxidation, results in material degradation, and places excess weight on the duct system requiring extra supports. Providing a separate cover or placing the ducts in a protected area on the roof is most often not practical.
There is, therefore, still need for a duct design that alleviates some of the problems associated with exposed duct work, without requiring changes or additions to the installation area.